The present invention relates to a vacuum circuit breaker and more particularly to a vacuum bulb which is suitable for use in a dirty and damp atmosphere.
A conventional vacuum circuit breaker is schematically illustrated in FIG. 1. The vacuum circuit breaker comprises an insulating vessel 1, a stationary contact 2 and a movable contact 3. The insulating vessel 1 provides not only insulation between the contacts 2 and 3, but also hermetic seal to maintain a high degree of vacuum, and is required to be compact and reliable. Therefore, the vessel 1 is usually made of glass or ceramics and formed in a cylindrical shape. The outer surface of the cylindrical vessel 1 has a relatively small creeping distance, so that when the vessel 1 is wet and contaminated leak discharge, which is a creeping discharge initiated by a leak current, along the surface is easy to occur, and, moreover, flashover may occur upon opening of the vacuum circuit breaker to render the vacuum switch incapable of interruption.
To improve the insulating strength of the outer surface of the vacuum bulb, the cylindrical vessel 1 is covered by an insulating outer cylinder 4 made of epoxy resin or the like and having a corrugated outer surface having alternate furrows and ridges, so that the creeping distance is increased. The epoxy resin may be directly coated or molded on the insulating vessel 1. In this case, the manufacture of the vacuum bulb is relatively simple, but the difference in coefficient of thermal expansion between the insulating vessel 1 and the insulating outer cylinder 4 causes internal stresses, which may cause breakage of the vacuum bulb or separation at the interface between the insulating vessel 1 and the insulating outer cylinder 4, leading to corona discharge. Alternatively, an intermediate layer 5 of soft resin may first be provided on the insulating vessel 1 and the resinous intermediate layer may be provided on the intermediate layer 5. The interposition of the layer 5 relieves the internal stresses, so that the breakage and the separation are avoided. However, because no inorganic filler is mixed in the soft resin, the intermediate layer 5 does not have sufficient mechanical strength and is liable to break due to impact which takes place during the closure and opening of the vacuum switch.
Moreover, use of resinous moldings as the insulating outer cylinder increases weight and size of the vacuum bulb, and cost of the mold and the resinous material.